Automatically shiftable transmissions require a supply of pressurized oil or fluid to operate. The pressurized fluid may be used for such functions as lubrication, torque converter operation, and engagement of torque transmitting mechanisms for gear ratio interchange. The fluid is typically stored in a main reservoir or main sump volume where it is introduced to a pickup or inlet tube for communication to a positive displacement hydraulic pump. The pump operates to pressurize the fluid for subsequent communication to the transmission. Therefore, the inlet tube must remain submerged in fluid during the operation of the transmission to avoid pump starvation. To this end, an adequate oil level is typically maintained within the main sump volume of the transmission during cold operation. However, at higher fluid temperatures, the fluid tends to expand and entrain an amount of air such that the volume of fluid within the main sump volume may increase significantly. This increase in fluid volume may interfere with moving parts within the transmission, such as a drive link assembly, causing a decrease in transmission efficiency and a possible increase in fluid aeration. An increase in fluid aeration above acceptable limits may lead to transmission malfunction. To maintain the proper fluid level within the main sump volume, an auxiliary sump volume may be employed to contain the excess volume of fluid at warm fluid temperatures and to communicate this excess volume of fluid to the main sump volume at cold fluid temperatures.